While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 nm). Consequently, operating voltages were very high, often >100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate at a much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light emitting layer (LEL) between the hole transporting layer and electron transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616, 1989]. The light emitting layer commonly consists of a host material doped with a guest material. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole injecting layer (HIL), a hole transporting layer (HTL), a light emitting layer (LEL) and an electron transporting/injecting layer (ETL). These structures have resulted in improved device efficiency.
Many emitting materials that have been described as useful in an OLED device emit light from their excited singlet state by fluorescence. The excited singlet state is created when excitons formed in an OLED device transfer their energy to the excited state of the dopant. However, it is generally believed that only 25% of the excitons created in an EL device are singlet excitons. The remaining excitons are triplet, which cannot readily transfer their energy to the singlet excited state of a dopant. This results in a large loss in efficiency since 75% of the excitons are not used in the light emission process.
Triplet excitons can transfer their energy to a dopant if it has a triplet excited state that is low enough in energy. If the triplet state of the dopant is emissive it can produce light by phosphorescence. In many cases singlet excitons can also transfer their energy to lowest singlet excited state of the same dopant. The singlet excited state can often relax, by an intersystem crossing process, to the emissive triplet excited state. Thus, it is possible, by the proper choice of host and dopant, to collect energy from both the singlet and triplet excitons created in an OLED device and to produce a very efficient phosphorescent emission.
The light emitting layer is typically composed of a host material and a dopant. However, recent advances have shown that the use of a LEL containing more than one host material may result in an improved electroluminescent device. The light emitting layer (LEL) is typically composed of a host material and a dopant. However, recent advances have shown that the use of a LEL containing more than one host material may improve performance of electroluminescent devices. For example, US 20020074935 discloses OLED with a LEL containing a phosphorescent dopant mixed with electron-transporting and hole-transporting co-hosts. U.S. Pat. No. 6,734,457, US2005123797, US2005121666 and US 2006134460 also teach OLED structures with a phosphorescent LEL with mixed electron-transporting and hole-transporting co-hosts. In particular, EP1661899 discloses OLEDs with a phosphorescent LEL with a mixture of a hole-transporting carbazole and a spirobifluorene compound as co-hosts.
The properties of the electron transporting layer are particularly important in determining the operational voltage of an electroluminescent device. Electron transporting layers containing mixtures or layered structures are often useful for electroluminescent device with low voltages. Devices with an electron transporting function made up of more than one layer have been shown in JP200338377, US20050025993, US20050019604, and US2005013388. In the situation where the ETL is directly adjacent to the cathode, it is sometimes alternatively called an electron-injection layer (EIL). If the ETL has a sublayer that is in direct contact with the cathode, then the sublayer is called an EIL and together they form an ETL/EIL/cathode structure. U.S. Pat. No. 6,639,357 discloses OLEDs with an ETL and an EIL. U.S. Pat. No. 6,396,209 discloses OLEDs with EILs with an electron-transporting compound and an organic metal salt adjacent to the cathode.
The use of triphenylamines with only one nitrogen atom as hole-transporting materials suitable for use as hosts for LELs in OLEDs is known. For example, CN1769269, JP2005085599, JP2008288344, Z. Jiang et al., Organic Letters, 11(7), 1503 (2009), Z. Jiang et al, Chem. Comm., 23, 3398 (2009), P. Shih et al., Advanced Functional Materials, 17(17), 3514 (2007) and C-H. Wu et al, Applied Physics Letters, 92(23), 233303 (2008) all disclose OLEDs with a triphenylamine host in the LEL. US2007003785 discloses benzidines with spiro substituents.
Benzophenones with spiro substituents are known as electron-transporting hosts for phosphorescent LELs; for example, see US20060208221 and US20090167166.
Notwithstanding these developments, there remains a need for new device structures that will result in phosphorescent electroluminescent devices having low operational voltage and high luminous efficiency.